1. Field of the Invention
The present invention relates in general to a biaxially stretched polyester film, and more particularly to a biaxially stretched, laminated microporous polyester film, which is non-transparent and has excellent flexibility so that it can be used in various applications, including printing, imaging, advertising and display.
2. Background of the Related Art
Polyester, mainly polyethylene terephthalate, has excellent physical and chemical properties, and hence, is widely used for polymer-processed products. Particularly, in view of development for use as a film, a porous polyethylene film is actively developed for use as substitutes for labels, cards, white boards, photo papers, imaging papers, and the like. A synthetic resin-based synthetic paper has excellent water resistance, hygroscopicity, dimensional stability and surface smoothness, as compared to a natural paper, and when printed, exhibits excellent gloss, sharpness and mechanical strength. As a white film for the synthetic paper, there is mainly used a polyolefin-based film, which is based on an olefin resin, contains an inorganic filler and a small amount of additives and includes a great number of micropores formed by biaxial stretching. Also, as the white film, there is mainly used a polyester-based film, which has a thermoplastic resin added to a polyester resin non-compatible with the thermoplastic resin and contains a great number of micropores formed by biaxial stretching.
There is well known a method for producing a microporous polyester film, wherein a thermoplastic resin non-compatible with a polyester resin is filled in a matrix at a suitable amount, and micropores are formed by a stretching mechanism. Examples of the thermoplastic resin used in this method include polyolefins, polystyrenes and the like. For example, GB Patent No. 1195153 discloses a method for producing a biaxially stretched film, in which 0.01–5% by weight of polyamide or polypropylene is added to polyethylene terephthalate. Moreover, U.S. Pat. No. 3,579,609 discloses a biaxially stretched film, which is based on polyethylene terephthalate and contains less than 40% by weight of a polymer material selected from a polyolefin polymer, such as polyethylene, polypropylene, polybutylene, polymethylpentene and the like, an ethylene vinyl acetate copolymer and polytetramethylene oxide. Furthermore, U.S. Pat. No. 3,640,994 discloses a biaxially stretched film, which is based on polyethylene terephthalate and contains 1–30% by weight of a polymer, such as polysulfone, polymethylpentene and the like, and 0.2–3% by weight of a light absorber. In addition, U.S. Pat. No. 3,944,699 discloses a biaxially stretched film, which contains 3–27% by weight of polyethylene or polypropylene added to polyethylene terephthalate.
GB Patent No. 1563591 discloses a polyester film, which contains 5–50% by weight of barium sulfate with an average particle size of 0.5–10 μm and also polyethylene as a foaming accelerator dispersed therein. EP 0044616, U.S. Pat. No. 4,368,295 and Japanese Patent Application Laid-Open No. Sho 64-02141 disclose a biaxially stretched, non-transparent or semi-transparent film comprising 0.5–100% by weight of at least one polyester resin, 0.05–50% by weight of at least one polyolefin resin, and at least one carboxylated polyolefin resin. U.S. Pat. No. 4,770,931 discloses a shaped article comprising a continuous polyester phase having dispersed therein microbeads of cellulose acetate which are at least partially bordered by void space. In the shaped article disclosed in this patent, the microbeads of cellulose acetate are present in an amount of about 10–30% by weight based on the weight of said polyester, the void space occupies about 2–50% by volume of the shaped article, the polyester has an I.V. of at least 0.55, the cellulose acetate has an acetyl content of about 28–44.8% by weight and a viscosity of about 0.01–90 seconds, and the microbeads has an average diameter of about 0.1–50 microns.
Korean Patent Publication No. 1994-0011167 and Japanese Patent Application Laid-Open No. 64-18631 disclose a monoaxially or biaxially stretched microshell polyester film with an apparent specific gravity of 0.4–1.3 and a hiding power of more than 0.2, which has a coating layer at one or both surfaces thereof. This coating layer contains at least one compound selected from an organic solvent-soluble thermoplastic polyester resin, an water-dispersible, sulfonated thermoplastic polyester resin, an alkyd polyester resin, acrylic-modified polyester resin, an organic solvent-soluble or water-dispersible vinyl resin, a polyisocynate compound, a blocked polyurethane resin, an epoxy resin, a silicon resin, a melamine resin and the like.
Korean Patent Publication No. 1996-0004143 discloses a porous polyester film with a porosity of less than 4% at a portion up to 3 μm in a thickness direction from the film surface and also an average porosity of 8–50% by volume. Moreover, Korean Patent Publication No. 2000-0023359 discloses a laminated microporous polyester film comprising a microporous polyester layer A with a density of 0.5–1.2 g/cm3, and at least one polyester layer B laminated on at least one surface of the microporous polyester film A, in which the polyester layer B contains white inorganic particles of 0.05–0.44 g/m2 and has a density of more than 1.1 g/cm3, which is at least 0.1 g/cm3 higher than that of the layer A. Furthermore, Korean Patent Publication No. 2000-0025195 discloses a white porous polyester film containing 60% by weight of ethylene terephthalate as a polyester resin, 5–35% by of a polyolefin resin with a melt index of 1.0–25 g/10 minutes, and 0.1–2.5% by weight of a non-ionic antistatic agent. Korean Patent Publication No. 2000-0027216 discloses an at least monoaxially stretched, multi-layered, white porous polyester film for use as a substitute for paper, which comprises an intermediate layer B made of an inorganic microparticle-containing resin layer comprising a resin mixture of 65–95% by weight of a polyester resin and 5–35% by weight of a polyolefin resin, and an elastomeric copolymer added at the amount of 0.3–2% by weight relative to the total weight of the resin mixture; and a surface layer A made of an inorganic microparticle-containing polyester resin layer, which is laminated on at least one surface of the intermediate layer B.
Recently, an inkjet printing method is rapidly applied to various devices including facsimiles, printers and plotters because of low noise, the possibility of high-speed recording, the easiness of full-color recording and particularly, low cost. Meanwhile, with high-speed and full-color trends of recording, there is also a requirement for a high-quality property in a recording sheet. For example, in order to increase the sharpness of printed images, even in the case of using synthetic paper or plastic film as the recording sheet, requirements for the hue b value and surface gloss of the recording sheet are often examined unlike the past.